The physiological significance of aortic smooth muscle (ASM) responses is evaluated from their effects on aortic impedances measured every 5 cm along the aorta. At each location, impedance spectra are obtained from aortic pressure, flow and diameter measurements using a cathether-tip instrument with three different sensors. Impedance spectra are plotted in a three dimensional plot with modulus, frequency and distance from aortic valves as coordinates. The effects of ASM responses produced by vasoactive agents are studied. ASM activity is quantitated from measurements of transverse distensibility all along the aorta. In electrical circuits a mismatch between source impedance (generator) and load frequently affects the efficiency of energy transfer from source to load. Assuming that similar conditions prevail in hydraulic systems, left ventricular efficiency, determined from external work and heat dissipation, will be studied as affected by ASM responses. Only the heat fraction dissipated via the coronary sinus flow can be measured directly. The percentage of the total heat dissipated via other routes will be determined by injecting blood with a known amount of "excess heat" into the left coronary artery. The fraction of excess heat recovered in the sinus should equal the fraction of myocardial heat dissipated via the sinus.